Our present invention relates to ceramic materials in general and, in particular, to semiconductive ceramic materials composed principally of strontium titanate, SrTiO.sub.3, and having a voltage-dependent nonlinear resistance. The ceramic materials according to our invention find a typical application in varistors known also as voltage-dependent resistors.
Parallel connections of varistors and capacitors have been used extensively for the absorption or suppression of abnormal voltages in electronic circuits. It is obvious, then, that devices possessing both varistor and capacitor functions can attain the same purpose with simpler circuit configurations. Our European Patent Publication No. 44,981, dated Feb. 3, 1982, describes and claims semiconductive ceramic compositions in a perovskite structure consisting primarily of SrTiO.sub.3, suitable for use in varistors capable of such a dual function. The dual-purpose ceramic varistors according to this prior art can favorably suppress abnormal voltages without the aid of capacitors.
The conventional SrTiO.sub.3 ceramic varistors, however, proved to be susceptible to voltage or current surges. In order to make them surge-proof and hence to widen their field of applications we have proposed improved SrTiO.sub.3 ceramics in our European Patent Application No. 82,106,421.9 filed July 16, 1982, now Patent Publication No. 70,540 dated Jan. 26, 1983. The improved SrTiO.sub.3 ceramics contain sodium monoxide, Na.sub.2 O, as a surgeproofing agent. The varistors made from the SrTiO.sub.3 -Na.sub.2 O ceramics can well withstand voltage and current surges, besides being capable of functioning not only as such but also as capacitors.
Although satisfactory for most practical purposes, the known SrTiO.sub.3 -Na.sub.2 O ceramic varistors have still proved to have one weakness in a limited field of applications. It is the temperature dependence of the varistor voltage (i.e., in the volt-ampere curve of the varistor, the voltage at which the current starts rising in magnitude). Varistors whose varistor voltages change less in a wider range of temperatures will find their way into electronic circuits of motor vehicles or the like, where they are expected to function properly in a temperature range as wide as from -40.degree. to +120.degree. C.